The relevant prior art in relation to such plug connectors is provided, for example, by EP 0 753 698 B1, EP 0 733 844 B1 and DE 203 19 959 U1 (US 2007/0284875 A1) which describe various applications and structural designs of known coupling devices.
In this respect, EP 0 733 844 B1 describes a coupling device for the rapid and releasable coupling of at least one plastic pipeline, comprising a housing part with at least one receiving opening for inserting the pipeline, and also a clamping ring which is arranged in the receiving opening and which, in order to lock the pipeline, interacts via an outer cone with an inner cone of the housing part, wherein, for the purpose of being able to release the pipeline, the housing part comprises, in two-part form, a base part and an insert part which is connected releasably to said base part and which incorporates the inner cone, and the insert part is connected to the base part via a snap-action form-fitting connection. Provision is made for the insert part to have an inner, encircling sealing lip so that it can bear sealingly against the outer peripheral surface of the pipeline, wherein the sealing lip extends obliquely inward in the direction of insertion prior to the insertion of the pipeline in such a way that it is somewhat elastically widened by the inserted pipeline and then bears sealingly.
EP 0 753 698 B1 describes a plug connection for coupling pressure-medium lines, comprising a housing part with a socket having a one-side opening for the insertion of a plug section formed in particular by a pipeline end, wherein at least one retaining element is arranged within the socket on the side facing the opening and at least one sealing element is arranged, starting from the retaining element, in the direction away from the opening, and, during the operation of inserting the plug section into the socket of the housing part, a retaining function is ensured first of all, and, in the further course of the insertion operation, sealing of the plug section with respect to the housing part is additionally ensured via the sealing element, wherein the housing part comprises a leakage path in such a way that, when the plug section is in an inserted position in which it is locked by the retaining element but not yet sealed via the sealing element, a perceptible leakage within defined limits is ensured. Provision is made for the leakage path to be assigned a valve element in such a way that, with the plug section in the inserted position in which it is not yet sealed via the sealing element, it is possible on the one hand for the pressure medium to pass outwardly via the leakage path but, on the other hand, dirt particles and the like are prevented from penetrating from outside into the interior of the housing.
DE 203 19 959 U1 (US 2007/0284875 A1) describes a plug connector for fluid lines, comprising a housing part with a plug socket for the fluidtight insertion of a tubular plug-in part, wherein a retaining element for locking the inserted plug-in part and a fluid seal for sealing said part are arranged in the plug socket, and wherein the housing part comprises, in two-part form, a base part and an insert part which is connected thereto via a snap-action form-fitting connection. Provision is made for the base part in turn to comprise, likewise in two-part form, a receiving part for the retaining element, the fluid seal and the insert part and also a coupling part for coupling the housing part to a fluid line.
At the same time, DE 203 19 959 U1 (US 2007/0284875 A1) also claims a plug connector for fluid lines that can have the above features, comprising a housing part with a plug socket for the fluidtight insertion of a tubular plug-in part, wherein, starting from a dirt seal on the mouth side as viewed in the direction of insertion, first a retaining element for locking the inserted plug-in part and subsequently a fluid seal are arranged within the plug socket, wherein a leakage path is formed in such a way that, with the plug-in part in a prelocking position in which it is locked by the retaining element but is not yet sealed via the fluid seal, a physically perceptible leakage within defined limits is ensured. Provision is made for the leakage path to be formed by depressions which are arranged on the outer periphery of the plug-in part and which, in the prelocking position, are arranged on the one hand in the region of the fluid seal and on the other hand in the region of the dirt seal.
In the case of one-part couplers (EP 0 753 698 B1, EP 0 733 844 B1), it is conventional for receiving parts together with retaining element and sealing ring to be inserted into a metal screw-in stub. The disadvantage with these plug connections is that the content of metal is very high and therefore the costs for material and machining are also correspondingly high.
Consequently, use has also been made of two-part designs having reduced metal content. In the case of these two-part plug connections, it is conventional for preassembled plastic receiving parts together with retaining element and sealing ring to be fastened to screw-in parts, as can be found in DE 203 19 959 U1 (US 2007/0284875 A1) in particular in the embodiment illustrated in FIG. 6 thereof. These known plug connections follow the basic principle of “retention before sealing”. The disadvantage of these plug connectors is that, apart from the main sealing ring which seals the pipe with respect to the receiving part, a further sealing ring has to seal the receiving part with respect to the screw-in part. This entails high costs on the one hand and on the other hand creates a point at which leakages can occur.